In connection with the invention, a complex measurement signal is understood to be composed of a number of individual measurements following one another in time. Concretely, a complex measurement signal is, for example, an echo curve, an envelope curve or spectral values. Echo curves or envelope curves are used for determining or monitoring the fill level of a fill substance in a container by using a travel time method. A spectrum or spectral values, which compose a spectrum, generally serve for the analysis of the composition of a medium. Field devices serving to register and/or influence process variables are often applied in process automation technology as well as in manufacturing automation technology. Measuring devices such as, for example, fill level measuring devices, flow measuring devices, pressure and temperature measuring devices, pH measuring devices, conductivity measuring devices, etc. are applied to register process variables and ascertain the corresponding process variables fill level, flow, pressure, temperature, pH value, and conductivity, respectively. For influencing process variables, actuators, such as valves or pumps, are used, via which e.g. the flow of a liquid in a pipeline or the fill level of a medium in a container is changed. All types of measuring devices and actuators are subsumed under the term ‘field devices’ used in connection with the invention. In relation to the invention in general it can be said that all devices, which are applied near to the process and deliver or process process-relevant information, are referred to as field devices. In addition to the measuring devices/sensors and actuators named above, units, which are connected directly to a fieldbus and serve for communication with a superordinated unit, such as e.g. remote I/Os, gateways, linking devices and wireless adapters or radio adapters, are also generally referred to as field devices. The Endress+Hauser Group provides a large number of such field devices.
If problems temporarily occur in a field device or in the process, in which the field device is applied, then it is important for service personnel to have measuring signals, which were ascertained during the problem, available, since the measuring signals can possibly include information concerning the irregularities. A diagnostic system for a field device in a process control system is described in U.S. Pat No. 7,516,043 B2, wherein a sensor is connected to a process control apparatus. Furthermore, a computing unit is provided, to which the data of the sensor are fed and which monitors the occurrence of a predetermined event in the process. If there is an event, the data before the occurrence of the event and after the occurrence of the event are stored in a memory unit for a predetermined time span.
In U.S. Pat. No. 7,516,043 B2, one dimensional measured values, such as pressure measurement values, are stored. Such a procedure is not applicable in a fill level measurement, which is based on the travel time principle, or in a spectral analysis. U.S. Pat. No. 7,516,043 B2 presents no solution for having information concerning the functional ability of the corresponding field device available, even at a still later point in time.